Substation, its types, design and construction explained in detail

SUB STATION

As the power demand increases in this economical generation of electrical energy, it becomes more common to have the bigger and bigger power houses. The power houses might be closer and might be far from load center; in any case it needs transmission network.

In the midst of power houses and the consumer many transformation need to be installed that transformation is called substation.

A substation is the system which converts one voltage level into on other voltage level and mostly it convert 11kv into 415v.

Normally the user do not want high voltage i.e. 11kv, so it need to be convert into 415v by the means of substation, so we can say that substation is the link between the generating stations and the consumer main.

Our project is limited to an office building, so a distribution substation also needs to be considered. The distribution substation converts the voltage to a small value for the use of small power consumer. Generally we use 3 phase 4 wire system for the distribution substation with 415v between the lines and 230v per phase.

i.                   DESIGN OF SUBSTATION

For the design of substation some important points need to be considered

Ø  It should be possible to have an easy and safe access for the maintenance and inception of different equipment.

Ø  The layout of the substation should be such that when the fault comes in power system it isolates the fault as quickly as possible.

Ø  By considering the capacity of substation, number of feeders etc, lay out of the switch gear is decided.

Ø  Power cable and control cable should be separate.

Ø  By considering the IP rating fire proof switch room and cable should be installed in order to safe from fire hazards.

ii.                 EQUIPMENT USED IN SUBSTATION

The substation consists of the following equipment.

1-Insulators                              2-Conductors                        3-Reclosing and LT switch gear

4-Circuit breaker                    5-Transformer                      6-Protective fuses and relays

7-switch board                        8- metering

1-INSULATORS

For backing of bus bar and live conductors insulators are used. For the low voltages up to 11kv insulators can be used in stack and can be mounted horizontally and vertically as per condition but for low voltages beyond that, insulators are mounted mostly in horizontal configuration.

2-CONDUCTORS

Conductors are of the two types for the substation.

1- Rigid buses of solid conductors

2-Strain buses of cable

These both types of conductor could be of Aluminum and Copper.

3-RECLOSING AND LT SWITCHGEARS

A recloser is a circuit breaker equipped mechanism, that trip the breaker after it has been opened due to faulty. We use recloser for the overhead distribution system relosers are used on a low voltage bus structure column. While, the switchgear is the combination of disconnect switches, fuses and circuit breaker used to isolate the equipment in the substation. The distribution of current on various places depends on the number of outlet required.

4-CIRCUIT BREAKER

Mostly vacuum circuit breaker is used for inside the building that’s why we used vacuum circuit breaker.

 Vacuum circuit breaker is a type of circuit breaker where arcing occurs in a vacuum. This is normally used for medium voltage application, for high voltage this technology also developed but it is not common. The opening and closing of current carrying contacts are associated to arc interruption which is take place in a vacuum chamber of the breaker which is called vacuum interrupter.  The pressure of vacuum is normally maintained to 10^-6 bars. The contacts are made up of cu and cr.

4.1-ADVANTAGES OF VACUUM CIRCUIT BREAKER

Ø  Life of vacuum circuit breaker is longer than the other type of circuit breaker.

Ø  Chances of fire hazard are almost negligible.

4.2-OPERATION OF VACUUM CIRCUIT BREAKER

The task of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero. At current zero this vacuum arc is extinguished and the conducting metal vapor is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero.

There are two types of arc shapes. For interrupting current up to 10kA, the arc remains diffused and the form of vapor discharge and cover the entire contact surface. Above 10kA the diffused arc is constricted considerably by its own magnetic field and it contracts. The phenomenon gives rise over heating of contact at its center. In order to prevent this, the design of the contacts should be such that the arc does not remain stationary but keeps travelling by its own magnetic field. Specially designed contact shape of vacuum circuit breaker make the constricted stationary arc travel along the surface of the contacts, thereby causing minimum and uniform contact erosion.

5-TRANSFORMER

The function of transformer in substation is to convert 11kv into 415v.in Pakistan mostly oil type transformer is used because of its large service life. Oil type transformer requires minimum maintenance. The main factors which is need to be considered for the installation of transformers are

A-     Initial costs

B-     Operating costs

C-     Reliability of supply

D-     Served load

6-PROTECTIVE FUSES AND RELAYS

Normally fuses are provided with the transformer and other protective devices. Its function is to protect the transformer.

Protective relays are also provided for the safety of power system. For the protection of transformer we use BUCHOLOZ RELAY.

When a fault occurs in a transformer or the short circuit occur in the transformer then the pressure inside the oil tank increases and oil starts moving towards conservators in this passage it also passes through the BUCHOLZ RELAY. The baffles in the relay get passed by the rushing oil and now the relay will operate and accordingly it trips the circuit breaker. Now we can remove transformer for the service.

7-SWITCH BOARDS

Meters, relays and control equipment are places in a switch board. Two meters i.e. essential and non essential are placed at the top of relay and bottom of relay respectively. The control equipments are normally placed between half way between top and bottom so as to facilitate their operation.

8-METERING

The energy meter incorporating maximum demand indicators is provided with the incoming HT supply. An ammeter and volt meter is also provided with the selector switch to shift across the lines are also fitted in the HT panel. An ammeter and one volt meter has been also provided with the LT lines.